Last_value
窗口函数doesn't
正常工作。
CREATE TABLE EXAMP2
(
CUSTOMER_ID NUMBER(38) NOT NULL,
VALID_FROM DATE NOT NULL
);
Customer_id Valid_from
-------------------------------------
9775 06.04.2013 01:34:16
9775 06.04.2013 20:34:00
9775 12.04.2013 11:07:01
--------------------------------------
select DISTINCT LAST_VALUE(VALID_FROM)
OVER (partition by customer_id ORDER BY VALID_FROM ASC) rn
from examp1;
当我使用LAST_VALUE
时,我会得到以下行:
06.04.2013 20:34:00
06.04.2013 01:34:16
12.04.2013 11:07:01
当我使用FIRST_VALUE
时,我会得到以下行:
select DISTINCT FIRST_VALUE(VALID_FROM)
OVER (partition by customer_id ORDER BY VALID_FROM DESC) rn
from examp1;
4/12/2013 11:07:01 AM
First_value
查询提供正确的输出。我希望从这些查询中获得相同的输出。为什么我有2 different results
?
答案 0 :(得分:12)
在分析函数中,您需要指定窗口范围。默认情况下它是between unbounded preceding and current row
,我认为这是不言自明的。
基本上,当您指定partition by customer_id order by valid_from asc
时会发生这种情况:
customer id
valid_from
valid_from
日期开始,以当前行valid_from
结束。last_value
,返回当前行的valid_from
。您需要做的是指定一个持续的范围:
16:53:00 SYSTEM@sandbox> ed
Wrote file S:\spool\sandbox\BUFFER_SYSTEM_38.sql
1 select last_value(VALID_FROM) OVER (
2 partition by customer_id
3 ORDER BY VALID_FROM asc
4 range between current row and unbounded following
5 ) rn
6* from t
16:53:21 SYSTEM@sandbox> /
RN
---------------------------------------------------------------------------
04-DEC-13 11.07.01.000000 AM
04-DEC-13 11.07.01.000000 AM
04-DEC-13 11.07.01.000000 AM
Elapsed: 00:00:00.01
答案 1 :(得分:6)
first_value
和last_value
有点特别,因为它们需要一个可以操作的窗口。
您需要添加ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
,如下所示:
select DISTINCT LAST_VALUE(VALID_FROM) OVER (partition by customer_id
ORDER BY VALID_FROM ASC
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) rn
from examp1;
请参阅文档:http://docs.oracle.com/cd/B19306_01/server.102/b14200/functions001.htm#i81407并特别阅读有关窗口的部分。不是接受窗口子句的函数的默认子句是RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
,它解释了为什么一个订单有效而另一个订单无效!